By David Alexander

WASHINGTON (Reuters) - Defense Secretary Leon Panetta pledged on Friday the Pentagon "will not break faith" with men and women in uniform as it deals with the challenge of cutting hundreds of billions of dollars in military spending.

Acknowledging a multitude of security threats -- from cyber attackers and nuclear proliferation to the wars in Afghanistan and Iraq -- Panetta told an audience at his formal swearing-in ceremony "we do not have a blank check from the American people" in dealing with the challenges.

"I really believe that we do not have to make a choice between fiscal discipline and national security," said Panetta, the former CIA director. "By setting priorities based on sound strategy, based on good policy, we can focus ... those resources we need at those threats of today and tomorrow."

With President Barack Obama and lawmakers on Capitol Hill locked in talks aimed at bringing Washington's trillion-dollar deficits under control, officials at the Pentagon are bracing for huge cuts in defense spending over the next decade.

Lawmakers seeking to influence a budget-cutting deal ahead of an August 2 deadline for raising the U.S. debt limit have floated proposals that could lead to defense spending cuts of $800 billion or even $1 trillion.

Some of the cutbacks being discussed could include a reduction in the number of U.S. troops as well as changes in the military's healthcare or retirement systems.

In speeches and interviews this week, Obama, Panetta's boss, reiterated his willingness to cut defense spending.

He told National Public Radio in an interview taped Thursday that protecting domestic programs for the vulnerable was a priority. "So a lot of the spending cuts that we're making should be around areas like defense spending, as opposed to food stamps," Obama said.

On Friday, Obama, who asked the Pentagon earlier this year to come up with $400 billion in spending reductions over the next 12 years, reiterated his willingness to cut defense by "hundreds of billions of dollars" as part of a balanced deal to reduce the deficit.

Panetta, who served as former President Bill Clinton's budget director, said the Pentagon had to "continue to be accountable to the American people for what we spend, where we spend it and what the results are."

"But I am confident ... that we can do this in a way that will strengthen us for the future, that will make us more effective, more efficient and that will not break faith with the men and women who serve this country," he said.

In sometimes emotional remarks, he echoed his predecessor, Gates, in promising to be "a tireless advocate" for service members and their families and to ensure U.S. forces have "what they need to accomplish their mission."

(Editing by Todd Eastham)